1. Field of the Invention
The present invention relates to a vehicle that can be raised on a cushion of air and be propelled by air, and which incorporates improved air delivery means for performing the lifting and propulsion functions. It is a quite different type of air cushion vehicle.
2. Description of the Prior Art
The present invention is a development from U.S. Pat. Nos. 3,948,344; 4,155,421; and 4,298,083 which are all directed toward flexible film air pallets for transporting relatively heavy loads across surfaces with minimal frictional resistance. The pallet of U.S. Pat. No. 3,948,344 is representative of the pallets in this group of prior patents. The air pallet compresses a single flexible film sheet which is fixed about its edges directly beneath a load or load bearing surface and a plenum chamber is developed between the sheet and the load bearing surface upon pumping in of air. The sheet is perforated within an area lying beneath the load. Air enters an inlet into the plenum. Air dispersion means disperse the incoming air in the plenum beneath the load, and the air discharges from the plenum through the perforations in the flexible sheet for defining an air cushion that raises the vehicle.
Initially, the plenum is collapsed. In order to initially form the plenum, air is pumped in to jack or raise the load off the supporting surface upon which it is resting, slightly pillowing but not completely ballooning the flexible sheet of the air pallet. Herein, pillowing of the flexible sheet occurs upon expansion of the plenum chamber sufficiently to jack up the load and to provide the optimum size "footprint" for the flexible sheet, that is the area of the flexible sheet still immediately above the support surface. The footprint should be sufficiently large to avoid tipping of the load, yet optimally small to reduce frictional resistance to movement of the pallet across the support surface. When the flexible sheet is ballooned, in contrast, the plenum has become too large, the flexible sheet becomes too rounded, and the footprint becomes too small so that the load may become unstable and tip. Furthermore, with a ballooned flexible sheet, too many perforations are raised off the support surface, and they blow air sideways, rather than down at the support surface, wasting air and increasing the cubic feet per minute (c.f.m.) of air required to raise the pallet.
A portion of the flexible sheet at the edges of the load is preferably not perforated so that during initial inflation of the plenum with the load pressing from above, all of the initial inflation increases the pressure in the plenum, jacks up the load and pillows the flexible sheet without air escaping through exposed perforations in the pillowed sheet. Once the load is raised far enough and the pressure in the plenum increases enough, the air escapes through the perforations and forms an air cushion beneath the pallet and then escapes to the sides of the pallet.
Other object support systems use a flexible perforated sheet or diaphragm disposed beneath a rigid surface with an air pressurizable plenum defined between the diaphragm and the rigid surface. See U.S. Pat. Nos. 3,416,626; 3,844,509; and 3,739,407. Air escaping through the holes in the flexible sheet supports the system. The present invention, however, is not merely concerned with support, but is also concerned with propulsion and with controlling the movement of the air lifted vehicle, including in some embodiments controlled forward and rearward motion, braking and turning.
Other examples of a flexible diaphragm disposed beneath a platform to support a pallet or the like are shown in U.S. Pat. Nos. 3,261,177; 3,276,222; 3,321,038; and 3,844,509. These show an annular plenum defined beneath a rigid platform by a flexible sheet or diaphragm. Air is delivered to the center of the annulus and beneath the flexible diaphragm and the air moves out from the center of the annulus and passes beneath the annular diaphragm. Even when the exit of air to the center of the annulus is through the flexible diaphragm, the air is delivered to the central area surrounded by the annulus, as shown in U.S. Pat. No. 3,276,222, and as is also discussed in above-mentioned U.S. Pat. No. 3,844,509.
Also well known are air cushion vehicles. Such a vehicle typically comprises an upper deck, a peripheral wall or skirt depending from the deck, defining an open bottom space below the deck, and air pressure generating means, such as a propeller, which pressurizes the space beneath the deck and within the confines of the skirt, eventually building sufficient pressure to raise the skirt of the vehicle off a supporting surface such as the ground for enabling the trapped, pressurized air to escape. Often, the bottom edge of the skirt is flexible to both confine the pressurized air and to deform in passing over short-depth voids and short-height obstructions. In a number of these vehicles, the same means which pressurizes the space beneath the deck also blows air off to the side of the vehicle for propelling it. With a single means for generating air pressure, such as a propeller, the air flow may be divided, with part being used to raise the air cushion vehicle and part being used to propel it. Examples of such air cushion vehicles are shown in U.S. Pat. Nos. 3,563,333; 3,429,395; 3,608,662; 3,777,842; 3,869,020; 3,262,510; 3,401,766; and 3,587,771. It is of interest to note that none of these air cushion vehicles has the underside of the space beneath its deck, i.e. its plenum, enclosed by a sheet or otherwise encumbers exiting air flow out from beneath the vehicle. Even the vehicle of U.S. Pat. No. 3,827,527, which has a dispersal membrane, nonetheless uses the unencumbered air cushion beneath the peripheral skirt of the vehicle to provide lift, as in other patents.
An air cushion vehicle with an open bottom requires high c.f.m., and relatively low pressure for raising loads and then for propelling them. It would be desirable to reduce the need for c.f.m. while keeping the required air pressure level low, and the invention permits this, as discussed below. As a result, with the invention, more of the air supplied by an air blower is available for propelling the vehicle and less is needed to lift it.
Known air cushion vehicles experience difficulties in traveling over surface obstacles or voids, such as dips, trenches, and the like. Air cushion vehicles can serve as boats moving over water, since there are no voids and no tall obstacles to movement, and any obstacles which the craft may run into, such as waves, are not rigid, whereby the air cushion vehicle moves over what the vehicle would view as a generally flat surface. An air cushion vehicle can travel over a solid support surface, such as land, only if the surface does not have too deep voids or too tall obstacles and does not too rapidly change its contour. The vehicle cannot be raised or tilted considerably or pass over too wide a void, like a culvert or trench, because too much of the air will escape from beneath the peripheral skirt at the open bottom of the vehicle and the vehicle will simply settle to the ground. Increasing the air pressure and c.f.m. beneath the air cushion vehicle would not completely resolve the problem, because when the vehicle is raised too much, there is too large an opening for escape of the air built up beneath the peripheral skirt. The prior art, therefore, has not yet provided an efficient air cushion vehicle for traveling over land or a rough surface. The same observation would apply to the supports or vehicles having an annular plenum, where the support is obtained through air escaping beneath the annular flexible diaphragm.
As any vehicle with an open space or area confined by its periphery, or by a skirt or by an annulus moves at more than minimal speed, the air just above the surface over which it is moving will move up into the open area surrounded by a skirt or an annulus, and this will generate drag. While it is unlikely that there will be sufficient air resistance to significantly resist forward motion of the vehicle, nonetheless there will be some air turbulence beneath the vehicle caused by the air passing beneath it, which may interfere with the progress of the vehicle. In contrast, a smooth bottom provided by a flatter, pillowed sheet or diaphragm beneath the vehicle will smooth out the rough air flow and reduce turbulence and reduce the adverse effects on the progress of the vehicle which would be caused by turbulence.
In transportation of freight or persons, it has often been found that a train of vehicles is the most cost efficient transport apparatus. Conventional air cushion vehicles require so much c.f.m. that the means which generates an air flow typically only generates enough air pressure and c.f.m. to raise and propel a single vehicle. Were a train of air cushion vehicles to be provided, each would require its own air pressure generating means. More efficient use of the c.f.m., even to the extent of having a single source of air pressure supply a number of vehicles in a train of the vehicles, would be more efficient.